1. Field of the Invention
The present invention relates to control of electric motors, and more particularly to a method of compensating for variations in the efficiency of electric motors used in the drive systems of plastics processing machines, such as injection molding machines.
2. Description of the Related Art
It is well known that electric motors produce heat during their operation. Usually the amount of heat produced increases with the increasing horsepower of the electric motor. This heat reduces the efficiency of the motor, resulting in less torque output for a given current input. Unless the control system includes means to compensate for this variation in motor performance, the performance of the mechanism driven by the motor will also be affected. As plastics processing machines increasingly use electric motors to power the drive systems of the machine, the problem of compensating for this variation in operating efficiency becomes more apparent.
In electrically driven plastics processing machines there may be as many as three or more high horsepower electric motors used to drive the primary mechanisms of the machine. For example, an electromechanically driven injection molding machine would likely have electric motors to power the clamp unit and injection unit (both translational and rotational movement of the injection screw). If the torque from the motors operating these system varies significantly, inconsistent molding machine performance will result; i.e., variation in clamp force, injection rate and pressure, plasticizing time and/or back pressure.
Cooling systems for electric motors have been proposed in an attempt to narrow the operating temperature range and thereby limit variations in motor performance. While the various concepts for motor cooling can provide some benefit, no single approach is fully effective for the wide range of operating environments. Another drawback to implementing a cooling system in injection molding machines is the extensive modifications often required to adapt the motors to air or liquid cooling; these modifications can be complex and expensive. A means of controlling motor performance without the expense of additional components is highly desirable.